The present invention relates generally to draw out units. More particularly, the present invention relates to a plug-in circuit breaker main or feeder disconnect for a draw-out unit.
The use of switchgears in electrical distribution systems is well known. The switchgear houses a plurality of draw out units, with each draw out unit housing one or more switching devices, such as motor controllers and circuit breakers, which are releasably interconnected to one or more busbars. Periodically, the draw out units are removed from the switchgear to allow for maintenance of the equipment. The switching devices within the draw out units make electrical contact with the busbars through a plurality of clip connectors which extend from the draw out unit. Each clip connector is electrically connected to an electrical conductor for connection to the switching devices of the switchgear.
In a motor control system, vertical bus bars which carry current to motor controller units are arranged vertically in drawout units. Sliding stabs usually plug into a housing with opposing prongs being biased toward each other on the sliding stabs to contact either side of the bus bars, whereby the electrical connection between the motor controller unit and the bus bars is maintained. Lugs on the sliding stabs are connected to bus straps extending from the lugs to the line side of a circuit interrupting device such as a circuit breaker. Bus straps also extend from the load side of the circuit interrupting device to the line side of a distribution power load circuit, such as a starter.
Clip connectors of the prior art are generally mounted within an insulative housing that is affixed to a frame of the draw out unit. Each clip connector is typically constructed of an electrically conductive material formed into a xe2x80x9cUxe2x80x9d shape. The two free ends of the xe2x80x9cUxe2x80x9d-shaped clip form contact arms, which extend within the clip housing and are aligned for electrical connection with a specific busbar. As the draw out unit is inserted into the switchgear, the busbar slides between the two contact arms, and the contact arms frictionally engage the busbar. Typically, only one clip connector attaches to each bus bar.
The design of the contact clip requires each contact arm to have a surface area in contact with the bus bar to reduce resistance to current flow and thereby prevent inefficient thermal losses. In the U-shaped contact clips found in the prior art, a contact surface is formed along the entire width of each of the two contact arms. Ideally, these contact surfaces remain parallel to the bus bar when the bus bar is engaged between the two contact arms, thus allowing the full width of the contact arm to be in contact with the bus bar. However, inaccuracies during manufacturing and mishandling during installation can cause deformities the contact arms and their contact surfaces. Such deformities allow only a small portion of the contact arm to actually contact the bus bar, resulting in an increase in resistance to current flow.
The design of the contact clip also requires that the contact arms exert a minimum compressive force (force per unit of area in contact with the bus bar) onto the busbar. This force is required to overcome the electromagnetic force generated by current flow in the contact arms, which tends to part the arms from the bus bar. Problematically, the required minimum compressive force is proportional to the amount of force required to insert the contact clips onto the busbar. In other words, clips having a high compressive force are difficult to slide onto the busbar. As a result, it is necessary to use a racking mechanism in order to develop sufficient insertion force to install the draw out unit into the switchgear. The need for a racking mechanism increases the equipment cost for switchgear installation and maintenance. In addition, the need for a racking mechanism necessitates that clearance space be provided for the racking mechanism within the switchgear. The additional clearance space increases the size and cost of the switchgear itself.
Load terminals are electrically connected to the load side of the distribution power load circuit and are typically positioned to allow an output cable to be connected to them in a vertical configuration. These load terminals are generally bushings positioned on the bottom of the distribution power load circuit and have an axially threaded terminal to which output cable connections are made. Alternately, output cables of a thinner diameter may have attachments on one end that allow them to be secured to load terminals with screws or bolts. One particular method of securing output cable to a load terminal, as described in U.S. Pat. No. 4,154,993 entitled xe2x80x9cCable Connected Drawout Switchgearxe2x80x9d, involves mounting circuit equipment on a rolling carriage such that the load terminals engage connectors on stationary cables when the carriage is rolled over the cable connectors. Another method described in U.S. Pat. No. 5,107,396 entitled xe2x80x9cCircuit Breaker Combined Terminal Lug and Connectorxe2x80x9d involves sliding a cable connector disposed axially on the end of a cable into a receiving slot on a terminal lug. Both of these methods of attaching output cable are such that the output cable depends vertically, and not horizontally, from the load terminals.
Output cable extending vertically down from the load terminals must be bent at two right angles thus forming an S-shape before passing through an exit port in the bottom of the drawout unit. Depending on the voltage class, output cable may be up to a few inches in diameter and relatively inflexible due to its construction. Consequently, the minimum bending radius of the cable may be large. Because of this large minimum bending radius of the cable, and because the output port through which the output cable passes is usually not directly below the load terminals, the cable must be bent at right angles twice to pass out of the drawout assembly. Bending the cable to form two right angles requires an appreciable amount of space more than a single right angle bend would require. Thus, the drawout assembly must be of a larger size than is really necessary to accommodate both right angle bends. Furthermore, as the number of output cables depending from the load terminals increases, an increasing amount of space is required at the bottom of the drawout to accommodate the cables.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a drawout box for a motor control center. In an exemplary embodiment of the invention, the drawout box comprises a drawout base having a first side edge, second side edge, third side edge, and fourth side edge, a first side having a first side periphery, a second side having a second side periphery, a third side having a first side edge and a second side edge, a fourth side having a first side edge and a second side edge, a top bar of the first side connecting the third side to the fourth side, a top bar of the second side connecting the third side to the fourth side, wherein the first side periphery is defined by the top bar of the first side, the first side edge of the third side, the first side edge of the drawout base, and the first side edge of the fourth side, and the second side periphery is defined by the top bar of the second side, the second side edge of the third side, the second side edge of the drawout base, and the second side edge of the fourth side, and further wherein the second side allows free access into the drawout box and the first side includes a connection for a drawout handle.
In another exemplary embodiment of the invention, a drawout assembly comprises the above-described drawout box, plus a terminal assembly mounted upon the drawout base, the terminal assembly including a bus brace, the bus brace having a base plate, an upper surface of said base plate supporting a plurality of lug engaging receptacles, a lower surface of said base plate including sections for receiving bus straps, a corresponding number of lugs for receipt within the plurality of lug engaging receptacles, each lug including at least one port for receiving an output cable, each port having an axis lying parallel to a plane of said base plate and substantially perpendicular to longitudinal axes of the sections in the lower surface of said base plate, and, a plurality of output cables extending from the lugs and through the second side of the drawout box.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.